The present invention relates to a lidar arrangement for measuring various atmospheric turbidities, for determining causes of turbidity of gases and for indicating a distance of a visibility obstacle in the atmosphere.
It has been known to perform by a lidar transmission-, extinction- and backscatter measurement in the atmosphere in order to identify gases or particles present in the atmosphere, to measure their concentration and distance from the lidar station. Such known applications have been described for example in the publication of V. E. Derr, "Estimation of the extinction coefficient of clouds from multiwavelength lidar backscatter measurements", Applied Optics, Vol. 19, No. 14, pp. 2310-2314 for examining clouds, of J. S. Randhawa et al., "Lidar observations during dusty infrared test-1", Applied Optics, Vol. 19, No. 14, pp. 2291-2297, for the backscatter- and transmission measurements of explosion of TNT by means of a lidar equipped with CO.sub.2 -or Rubin-laser, and of D. K. Kreid, "Atmospheric visibility measurement by a modulated cw lidar", Applied Optics, Vol. 15, No. 7, pp. 1823-1831. In addition, the measurement of absorption and/or scattering of light in the atmosphere is disclosed for example in DE-OS 23 51 972 and DE-OS 23 28 092.
In the above mentioned patent publications a lidar is disclosed whose laser beam is transmitted via a transmission optical system. The transmitted laser radiation is absorbed and scattered in the atmosphere. A reception optical system whose aperture angle corresponds approximately to the aperture or apex angle of the transmitted laser beam, measures scattered light. Since the wavelength of the laser is known, from the intensity of the scattered light, the known dispersion cross-sections of different gases and particles as well as from the spectral absorption, the extinction coefficients of the atmosphere and thus the density of admixed gases and particles can be determined.
Known is also a lidar of a different kind wherein two laser beams of neighboring wavelengths are used whereby one of the wavelengths is subject to a particularly strong absorption or scattering by a gas to be determined while almost no absorption or scattering by this gas is exerted on the other wavelength. The "unaffected" backscattered laser radiation serves as a reference for the absorption of the gas to be determined in the atmosphere.
Due to the fact that the intensity of the backscattered light decreases with higher than square power of the distance of the scattering volume from the lidar, there are employed electronic gating circuits which evaluate only a certain distance range. As a rule, the lasers transmit short radiation pulses in the magnitude order of several tens nanoseconds; of course, modulated continuously operated lasers have been also employed.
The known lidar embodiments are suitable for measuring extinction coefficients of the atmosphere in general, and for measuring the extinction of an admixed gas or particles. The disadvantage of the prior art lidars is the fact that they are unsuitable to distinguish in an unambiguous and fast way different kinds of atmospheric turbidities, such as for example snow from fog, rain or solid visibility obstacles.